Batio3-mgo-nb2o5 dielectric ceramic compositions and method

ABSTRACT

BISMUTH-FREE, DIELECTRIC CERAMIC COMPOSITIONS CONSISTING ESSENTIALLY OF FROM APPROXIMATELY 97 MOL PERCENT TO APPROXIMATELY 82 MOL PERCENT BATIO3, THE BALANCE BEING MGO AND NB2O5 IN A MOL RATIO OF FROM ABOUT 1:1 TO 1:2, MAY BE FIRED AT TEMPERATURES IN THE RANGE FROM ABOUT 1300*C. TO 1400*C. TO PRODUCE USEFUL DIELECTRIC CERAMIC BODIES INCLUDING MONOLITHIC CAPACITORS. THE RESULTANT BODIES HAVE RELATIVELY LOW TEMPERATURE COEFFICIENTS, LOW DISSIPATION FACTORS, AND VARY IN RELATIVE PERMITTIVITY FROM ABOUT 300 TO ABOUT 2000. THE PERMITTIVITY MAY BE CONTROLLED BY ADJUSTING THE MOL PERCENTAGE OF MGO AND NB2O5 IN THE COMPOSITION.

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l 20m/@M205 v INVENTOR. TRUMAN C. RUTT ATTORNEY United States Patent Otiice 3,788,867 BaTi03-Mg0-Nb205 DIELECTRIC CERAMIC COM- POSITIONS AND METHOD Truman C. Rutt, Niagara Falls, N.Y., assignor to N. L. Industries, Inc., New York, N.Y. Filed Nov. 1, 1971, Ser. No. 194,457 Int. Cl. C04b 33/26, 35/46 U.S. Cl. G-73.31 8 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THB INVENTION This invention relates to dielectric compositions and is particularly concerned with dielectric ceramic compositions principally composed of barium metatitanate, with dielectric ceramic bodies formed therefrom, and with processes for the production of such compositions and bodies.

In the electronic industry there is a large demand for ceramic capacitors, because, in general, their cost is low and they have both good volumetric eiciency and high temperature usefulness, as well asinherent reliability. Most such ceramic capacitors are principally composed of barium metatitanate with relatively small amounts of other oxides, and compositions for capacitors can be obtained with very high permittivities. It has been found, however, that the dependence of permittivity on temperature is quite high in most such compositions. Although dielectric ceramic compositions and bodies principally composed of barium metatitanate and containing bismuth oxide as an additive or modifier have been found to have only relatively small variations in permittivity 'with temperature variations over a rather wide working temperature range, i.e. a low temperature coeicient, such compositions are usually undesirable for making monolithic capacitors since the bismuth reacts with palladium at elevated temperatures. This substantially precludes the use of the latter as internal electrodes in such capacitors and necessitates the use of more expensive noble metals as electroding materials in forming such capacitors. Hence, there has developed a need for dielectric ceramic compositions with reasonably high permittivities and now temperature coetiicients which do not contain bismuth compounds.

SUMMARY OF 'I'HE INVENTION It has been found that bismuth-free, dielectric ceramic compositions which consist essentially of from approximately 97 mol percent to approximately 82 mol percent barium metatitanate (BaTiOg), the balance being magnesium oxide (MgO) and niobium oxide (NbZOB) in a mol ratio of from about 1:1 to about 1:2, may be tired within a relatively wide range of temperatures to give dielectric ceramic bodies which have relatively low temperature coefficients and low dissipation factors, and which can be made to have permittivities within a considerable range.

3,788,867 Patented Jan. 29, 1974 More particularly, the present invention provides a process for producing dielectric ceramic bodies characterized by having, at 25 C. and 1 kHz, a dissipation factor (tan of less than .015 and a relative permittitivity of from about 300 to about 2000, with a variation from the permittivity at 25 C. of no more than about :1 -16% through the temperature range from 40 C. to 145 C. The process consists essentially in forming a mixture of BaTiO3, MgO, and Nb205, all in finely divided form, the mol ratio of the MgO and Nb2O5 present being from about 1:1 to about 1:2, and the BaTiOa constituting from approximately 97 mol percent to approximately 82 mol percent of the mixture, forming bodies from said mixture, and tiring said bodies in the temperature range from about 1300 C. to about 1400 C. In such dielectric bodies the permittivities vary with the mol percentages of the MgO and Nb2O5 present in the mixture.

SHORT DESCRIPTION OF THE DRAWING The drawing is a ternary diagram of a portion of the system BaTiOg-MgO-NbzOf, showing an area within which compositions are within the scope of this invention and denoting a number of such compositions.

DESCRIPTION OF THE 'INVENTION Hereinafter in this specification and the appended claims, percentages and ratios are molar unless otherwise indicated and permittivity tigures represent the permittivity relative to vacuum.

In the immediately following paragraph there is described a procedure by which bodies of a dielectric ceramic composition according to the invention may be produced.

BaTiO3, MgO, and Nb205, in powdered form, were blended together in the desired proportions. The ingredients were all finely divided, about 1 to 1.5# in size, and a small amount of alcohol was used to moisten the mixture during blending. After drying the blended mixture, about 5 wt. percent of a 5 wt. percent aqueous solution of polyvinyl alcohol was added and blended in to serve as al temporary binder. Ceramic test discs were formed from the mixture by pressing at about 700 kg./ cm?. The resultant discs, approximately 1.27 cm. in diameter and 1 mm. thick, were red, in air, at a temperature in the range from about 1300 C. to about 1400 C. for two hours and were thereafter electroded to form test capacitors by coating the opposed faces with a commercial electroding composition and retiring in air at about 815 C. for one hour.

The following table sets forth, as examples, a number of compositions within the scope of the invention and the tiring temperature employed in forming the test discs for each example. The listed compositions are indicated on the ternary diagram of the drawing.

TABLE A Composition in percent Firing tempera- (MgO, MgO ture, BaTiO; Nb205) NbzO C.

Composition in percent Test discs prepared substantially in accordance with the procedure set forth above and employing the compositions and tiring temperatures specified in the examples in Table A were tested at 1 kHz. to determine their relative permittivities (e) and dissipation factors (tan at temperatures in the range from 40 C. to 145 C. In the following table the relative permittivity at 25 C., the maximum plus and minus variations therefrom within the specified temperature range, and the dissipation factor at 25 C. are given for each said example. In most instances the figures in the table are averages of the readings from a plurality of discs.

TABLE B Permittivity Max. and Dissivariation, percent patlon factor, 25 C (-l') tan As indicated above, the dielectric ceramic compositions of the present invention are particularly useful, because of the relatively low temperature dependence, low dissipation factor, and the ease with which the permittivity 0f bodies made therefrom can be controlled, in forming monolithic capacitors. They are, as shown below, useful in the production of such capacitors even when palladium is used for the internal electrodes. The following example describes such use.

EXAMPLE 42 Sheets approximately 0.05 mm. thick were cast from a mixture of the composition of Example l0` with an ethyl cellulose base vehicle. On portions of the sheets there was deposited, by screen-printing, a thin film of a commercial palladium electroding paste. By pressing at about C. and 280 kg./cm.2 there was formed a composite body comprising eleven layers of the cast composition, all but the top layer having the printed lm thereon, and with the films on alternate layers extending to opposite faces of the body. The body was heated to yburn out carbonaceous material and was then tired at 1370 C. for two hours. After cooling, electrode connectors were applied in conventional manner to the ends of the fired body to bond the exposed internal electrodes.

The resultant monolithic capacitor, which was about 7 mm. square, had essentially the same dielectric properties as the test discs made from the composition of Example 10. The capacitance of the unit was 29 nanofarads.

In preparing the compositions of the present invention it is desirable, as pointed out above, to have the BaTiO3 and the oxides of magnesium and niobium in very fine form. So far as the BaTiOg is concerned, it is preferred to use one having an average particle size of from lp. to 1.5M and a fine crystallite size, since the use of BaTiO3 having a large crystallite size tends to increase the variation of permittivity with temperature between about C. and 130 C. in bodies made therefrom. The niobium and magnesium oxides preferably have an average particle size of 1p. or less and lit; is preferred to have no particles present in the powdered mixture larger than about 2n. The oxides may be relatively pure commercial grades since very small amounts of impurities will not appreciably affect the results.

It will be noted from the data set out above that within the relatively wide firing temperature range from about 1300 C. to about 1400 C. (2375 F. to 2550 F.) the dielectric characteristics of bodies produced from a particular composition are quite similar and in most cases relatively close. This is a great practical advantage since many dielectric ceramic compositions are quite sensitive to variations in firing temperature and consequently, very precise firing control is required.

It will also be noted that, within the scope of the invention, variation only of amounts of MgO and Nb205 used as additives permits the obtaining of dielectric ceramic bodies with permittivities ranging from approximately 300 to approximately 2000 from the same ingredients. This makes easier the production of dielectric ceramic bodies having a wide permittivity range but retaining a low temperature coeliicient and a low dissipation factor. In fact the present invention makes it possible to easily produce a product line of capacitors which are of identical size but widely varying capacitance, thus facilitating their application to circuit boards and the like.

What is claimed is:

1. A process for producing dielectric ceramic bodies characterized by having, at 25 C. and l kHz., a dissipation factor (tan of less than .015, and a permittivity of from about 300 to about 2000, with a variation from the permittivity at 25 C. of no more than about i16% through the temperature range from 40 C. to 145 C., which comprises forming a mixture of BaTiO3 MgO, and Nb205, all in finely divided form, the molar ratio of MgO to Nb205 being within the approximate range from 1:1 to 1:2, and theBaTiOa constituting from approximately 97% to approximately 82%; forming bodies from said mixture; and firing said bodies in the temperature range from about 1300,o C. to about 1400 C.; the permittivity being controlled by varying the total of the percentages of MgO and Nb205 from about 3 to about 18.

2. A process as set forth in claim 1 in which the particle size of the materials in said mixture is not larger than about 2p..

3. A dielectric ceramic composition consisting essentially of from about 82% to about 97% BaTiO3, the remainder being MgO and Nb205 in a molar ratio from approximatelylrl to approximately 1:2.

4. A dielectricLceramic composition as set forth in claim 3 wherein the particle size of the ingredients is not larger than about 2u.

5. A dielectric ceramic composition as set forth in claim 3 wherein the MgOzNb2O5 ratio is approximately 1:1.

6. A dielectric ceramic composition as set forth in claim 3 wherein the MgO:Nb2O5 ratio is approximate- 1y 2:3.

7. A dielectric ceramic composition as set forth in 6 claim 3 wherein the MgO:Nb2O5 ratio is approximately 1:2.

8. A body of the composition as set forth in claim 3 having, when fired in air at a temperature of from about 1300 C. to about 1400 C., a permittivity, at 1 kHz. and C., in the range from about 300 to about 2000 and a variation from the permittivity at 25 C. of no more than about 146% through the temperature range from C. to 145 C.

References Cited UNITED STATES PATENTS 2,801,181 7/1957 Das Gupta 106-39 3,243,315 3/1966 Markarian et al. 317-258 UX 3,235,939 2/1966 Rodriguez et al. 317-258 UX 3,237,066 2/1966 Martin et al 317--258 FOREIGN PATENTS 693,455 1953 Great Britain 317-258 20 HELEN M. MCCARTHY, Prima-ry Examiner U.S. C1. X.R. 317-258 UNITED STATES PATENT OFFICE CERTIFICATE .OE CORRECTION Patent No, 3, Dated January Inver'ltor!) Trliman C. Rutt It is certified that error appears in the above-identified patent and Chat .SBi,d Le1LeLS, Patent- .le herbv,cQITected as shown below:

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C. 'MARSHALL DANN MCCOY M. GIBSON JR.

Comrn xssioner of'Pate'nts Attestng Officer- ORM PCT-1050 (1o-59) uscoMM-Dc Goan-P69 U.5. GOVERNMENT PR|vNTlNG OFFICE Z |969 @-356-33 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3, 788, 86.7 Dated Jam-131729 1974 I'nVexItord)A v Truman C. Ratt l It is Certified that error appears in the above-identified pate'nt and thaiasailiLeLLrers. Patent-arafhexghyr3917er:ted as shown below:

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(SEAL) Attest:

MCCOY M. GIBSON JR. c. (MARSHALL DANN Attestng Officer v ConuIIssioner offPatents ORM '30050 (1069) I uscoMM-Dc Goan-Pes U.s. GOVERNMENT PRINTING OFFICE Z 195 0-365-33 UNITED STATES PATENT OFFICE Y CERTIFICATE F CORRECTION Y Peten: No. 3,788,867 f Detedv Janarv 29,' 1974 lnvetolf) I Tlliman C. Ruft It is certified that errr appears in th above-identified patent and thaL vvsaidELeELers Parenteargehereby,egrgrected as shown below:

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